I highly doubt you'll ever see a 1GHz factory OC'ed HD4890, at least not with full availability. I could see some "limited edition" cards, on the order of 1-5k units total. Kind of like a teaser or marketing gimmick.

The process by which to bin the chips at such high speeds necessary for production and the costs to fully test and ensure that the cards are absolutely stable at all times would be highly excessive. This would lead to a final product that is horribly overpriced and most likely lacking in any further OC'ing abilities.

An interesting option would be an HD4890 version that is OC'ing sanctioned and supported by the manufacturer. This model would have specially binned chips that are known to have quite a bit of headroom, but are just modestly OC'ed from the factory. This would leave the end user to extract the most out of the GPU, by whatever means possible.

I don't see why anyone would pay a premium for these types of aggressively "OC'ed from the factory" cards, as your luck is just as good messing around with a stock clocked model. But there is always a market, however small, and usually some manufacterer will try and serve it...

Actually, the final version (as soon as they get enough binned that can do the speeds) is 1.1Ghz from AMD using TEC Cooling. Idk if any brand names will pick it up but I can see 1.2-1.3Ghz on good TEC.